I just upgraded my ballast from an older T-12 to T-8, but the bulbs I bought are a bit too bright. They are daylight, and I knew they would be bright. But I figured the old, yellowed plastic sheet which sets below the bulbs would dim them enough. It doesn't.

So is there any harm in running just one bulb?

Since with this new ballast you can run one bulb, it would seem that it would be an okay thing to do.

I assume the voltage will be the same. But the ballast, as I read on the web, lowers the current. Will it run more current into the bulb and shorten its life? Obviously I'm clueless about this. I rely on forums for help and see the moderators here are great!

Actually I don't need to do much home repair, though I do all that is needed myself. So when I went to sign up to join the forum, I was surprised that I'd already joined before! Must have been for my toilet. LOL

Though it is a bit of a tease... I mean, since I'd only be running one bulb, the electricity I save might pay for the replacement bulb!

My other options are to just replace the new bulbs, put paper under them to dim the light, or not use the old T-12 fixture setting beside it. The problem with the latter is that I have a number of T-12 bulbs.

Your bulbs should have a number with a K (Kelvins) at the end of it. (3500K, 4100K, 5000K are common)
That is your color temperature. The lower the number, then "warmer" or more yellow the light. The higher the number, the "cooler" or whiter the light. I believe that as your Kelvins increase on fluorescent bulbs, your Lumens (amount of light) decreases. But your brain perceives the whiter light as brighter.
So try 2 different temperatures of bulbs. Lets say 2 3000K tubes, and 2 5000K bulbs and see if you're happier with your light.